Journal of Paleontology, 95(2), 2021, p. 298–304 Copyright © The Author(s), 2020. Published by Cambridge University Press on behalf of The Paleontological Society. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 0022-3360/21/1937-2337 doi: 10.1017/jpa.2020.96 Articulated trilobite ontogeny: suggestions for a methodological standard
Nigel C. Hughes,1,2 Jonathan M. Adrain,3 James D. Holmes,4 Paul S. Hong,5 Melanie J. Hopkins,6 Jin-Bo Hou,1 Alessandro Minelli,7 Tae-Yoon S. Park,8 John R. Paterson,9 Jin Peng,10 Mark Webster,11 Xi-Guang Zhang,12 Xing-Liang Zhang,13,14 and Giuseppe Fusco7
1Department of Earth and Planetary Sciences, University of California, Riverside CA 92521, USA
Abstract.—In order to maximize the utility of future studies of trilobite ontogeny, we propose a set of standard practices that relate to the collection, nomenclature, description, depiction, and interpretation of ontogenetic series inferred from articulated specimens belonging to individual species. In some cases, these suggestions may also apply to ontogenetic studies of other fossilized taxa.
Introduction (Sánchez, 2012). As a result, interest in describing and interpret- ing ancient ontogenetic series has burgeoned. Recent years have seen progress in describing ancient ontogenies Investigations of fossilized ontogenetic series can be aided by in ways that can be compared with those of living taxa, even at the defining standard analytical practices, including evaluation of the level of patterns and mechanisms of developmental control. By limitations that fossilization places on our ability to interpret them. conducting morphometric analysis of appropriate data sets Here, we review concepts and procedures relating to the descrip- derived from fossils it is now possible to move beyond simply tion and interpretation of articulated trilobite ontogeny. Although describing sequences of ontogenetic stages, and to address ques- the focus of this contribution is on articulated trilobites (a general tions of high interest for evolutionary developmental biology. overview of trilobite ontogeny can be found in the legend of This provides insight into how developmental processes evolve Figure 1), many of these issues apply generally in studies of and how such processes affect the evolution of organismal ancient ontogeny, in particular in other arthropod or arthropod- body patterning. As such studies progress in number and taxo- related taxa. Our aim is to highlight methodological standards nomic scope, it is becoming possible to assess variation in devel- that may increase the comparative value of future studies. opmental trajectories among and across clades, and thus to move beyond the typological approach dictated by rare exemplars. It is Need for a standard approach increasingly apparent that even the ancient fossil record can reveal subtle patterns of microevolutionary-scale variation that has The formulation and application of a set of standard practices potential for insights into how aspects of body patterning evolved and minimal requirements in quantitative studies of ancient 298 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.226, on 01 Oct 2021 at 19:15:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/jpa.2020.96 Hughes et al.—Standard for trilobite ontogeny description 299
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Figure 1. (1) Ontogenetic hypothesis of the alternative segmentation pathways of a polymorphic species across several postembryonic stages (s0–sn) or instars. Different morphological conditions (morphs, m), here based exclusively on the segmental pattern, are indicated as mXY, where X is the number of thoracic segments and Y the number of pygidial segments. Trunk segment addition initially proceeded at the pace of one or two segments per stage until stage s2, producing a poly- morphism in the number of trunk segments from stage s1 onwards. From stage s2 onwards, trunk segment addition proceeded at the pace of one segment per stage for all individuals, until the end of the anamorphic phase, at stage s4 for some specimens (m45) or at stage s5 for others (m44 and m43). Thoracic segment release pro- ceeded at the pace of one segment per stage and ended at stage s5 for all specimens. Morphs with five thoracic segments are holaspids (i.e., specimens that have reached a stable number of thoracic segments). Lines connect the succeeding morphological conditions of individuals across stages. The thickness of the lines con- necting the morphs reflects the frequency of the different morphs at each stage. (2) Misleading representation of the ontogenetic hypothesis in (1), where morphs (segmental condition) and stages are confused, and the former are seen as sequential meraspid and holaspid stages. Note the apparent reabsorption of one segment among some successive ‘stages.’ (3) One among several possible “correct” representations of the ontogenetic hypothesis in (1). This is correct only if the legend explains that there is variation in the pattern of segment addition within the sample and thus in the number of pygidial segments represented at several stages, and that the ontogenetic pattern depicted represents only one (possibly the more common) among alternative patterns. Parts (2) and (3) adopt a graphic style that has become common since Hughes et al. (2006), but other schematizations are possible. Colors highlight the three main body regions in trilobites: cephalon, blue; thorax (with dorsally articulated segments), light gray; pygidium (with conjoined segments), pink. The thorax and pygidium together form the trunk. During post-embryonic development, new segments were added subterminally, just in front of the terminal piece—triangle in (2) and (3). During the first post-embryonic stages (five stages shown in this schematic), the most anterior pygidial segment was progressively incorporated into the thorax region. This process, which consisted of the formation of a functional articulation in the posterior of that segment, is called “release.” Trilobite post-embryonic development is traditionally divided into three main periods: protaspid (not represented in the figure), meraspid (a period of segmental accretion in the thorax), and holaspid (a final period with a segmentally stable thorax). Alternatively, it can be divided into two phases: anamorphic (a phase of subterminal trunk segment addition) and epimorphic (a phase with a stable number of trunk segments). The presence of an anamorphic phase followed by an epimorphic phase qualifies trilobite development as hemianamorphic. In the specific case depicted, the onset of the epimorphic phase (termination of trunk segment addition) preceded the onset of the holaspid period (termination of thoracic segment release). However, this was not the case for all trilobite taxa. A more detailed account of trilobite ontogeny can be found in Hughes et al. (2006).
ontogeny offers several potential benefits. Firstly, it may clarify (including section thickness) from which the specimens have the methodology of investigation in order to ensure that a com- been derived is of critical importance for inferring the span of mon descriptive framework is applied among different studies time over which specimens in the sample accumulated. It is and taxa. This, in turn, facilitates the extraction of comparative necessary to note whether a sample comes from a single bed information from individual case studies, and thus may enhance (sensu Patzkowsky and Holland, 2012), from a series of similar understanding of the evolution of development. Secondly, a bedsets, or from units representing different depositional condi- standard protocol helps highlight the limitations of information tions. Blending of data from specimens occurring in different and interpretation that fossilization imposes. This aids authors, beds need not invalidate an analysis, but blending of data does reviewers, and editors in insuring that the various strengths place important constraints on the interpretation of patterns and weaknesses of further studies are immediately evident. revealed. Conversely, where information on precise stratigraphic occurrence is available, it offers valuable opportunity to exam- Outline and caveats of a standard study ine how variation is partitioned among collections with modest environmental or temporal differences. Information on lithology The nature of the sample.—Assessment of the sample’s bears on the degree of compaction witnessed in the sample and characteristics is an essential first step in evaluating and should be noted (e.g., calcareous mudstones are commonly less interpreting patterns of variation seen among specimens compacted than claystones). In addition, the cuticular condition belonging to a single species. of the specimen (testate, internal mold, external mold, etc.) may Any study must report repository information and comment also bear on measurements obtained. on the curation state of the material considered. Preferably, an For certain studies, the frequency of occurrence of exem- official registration/catalogue number should be available for plars attributed to any given stage or specific morph is relevant each specimen, and details of these provided as supplementary for testing alternative hypotheses regarding the ontogeny and/or material to published studies. Ideally, each specimen analyzed demographics of a species. In such cases, consideration of is both identified and stored individually, so that it can be recov- whether specimens can be determined to be exuviae or carcasses ered easily when next needed. If that is not the case, readers is of importance because this strongly affects the expected fre- should be informed. In addition, the systematics of the species, quency distribution under each hypothesis (Sheldon, 1988; including a clear diagnosis, must be included in the study if not Hartnoll and Bryant, 1990). However, making such a determin- already published. The rationale for uniting presumptive juven- ation, even in cases in which the exoskeleton remains articu- ile and adult specimens in the same ontogenetic series should lated, is commonly challenging. Only in exceptional cases can also be made explicit. Photographs of specimens should clearly the majority of a sample’s specimens be unambiguously illustrate relevant features and best results are commonly assigned to either category. achieved by applying standard paleontological photographic Coming to the dataset itself, the vagaries of fossil preserva- techniques, such as coating with ammonium chloride sublimate tion and recovery mean that often not all specimens containing or magnesium oxide (e.g., Feldman, 1989). Arrows indicating valuable information are complete in all characters of interest. the thoracic/pygidial boundary can be helpful, especially for Accordingly, sample size for each kind of measurement species with a homonomous trunk condition (Hughes, 2003). acquired should be specified, and will likely vary within the Attention should be paid to the geological context of the dataset. For example, it is necessary to state that out of N speci- material analyzed. This involves documentation of the site infor- mens available, the number of thoracic segments could be mation and number of specimens collected, and should also counted confidently on X and the number of pygidial segments include discussion of key geological indicators concerning the (often harder to determine) on only Y out of N (or X), or that size sample. For example, analysis of the stratigraphic interval measurements, such as body length and cephalic width, were
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available for W specimens, whereas only for Z specimens was it work of morph recognition. If this critical distinction between possible to obtain more comprehensive landmark-based mor- morphs and stages is overlooked, an ambiguous ontogenetic ser- phometric data. ies reconstruction can result (e.g., Dai et al., 2017, fig. 7, in Studies that contain morphometric data should also include which all morphs were presented as sequential stages). estimates of measurement error in specimen size and, where The distinction between description and interpretation may relevant, shape. The extent to which taphonomy affects morph- be reflected in the terminology adopted. In trilobite ontogeny, ology depends on the preservational quality of the specimens degree constitutes a morph that is defined by the number of thor- considered (as noted above). As with blending data from differ- acic segments. Degrees are generally referred to only during the ent beds, the taphonomic modification of form need not invari- meraspid period of development (e.g., a “degree X meraspid”). ably exclude biologically informative studies, but the effects of Because the meraspid period is a developmental phase, the taphonomic modification should be carefully gauged. For terms degree and meraspid are categorically distinct, and not example, the shapes of articulated but compressed complete spe- necessary coupled. For instance, in Aulacopleura koninckii (Bar- cimens (e.g., Hong et al., 2014; Holmes et al., 2020a) are more rande, 1846), specimens with 18 thoracic segments (i.e., degree strongly influenced by taphonomic factors than those of exquis- 18 specimens) include both meraspid individuals that would sub- itely preserved silicified sclerites (e.g., Webster, 2015), and so sequently attain 19, 20, 21, or 22 thoracic segments, and the the biologically meaningful questions that can be asked of holaspids for which 18 was the mature thoracic segment number such materials are necessarily different. It is key to ensure that (Fusco et al., 2004)—these could logically be referred to as a biologically informative question can be assessed realistically “degree 18 meraspids” and “degree 18 holaspids,” respectively. given the material available. Estimates of measurement error In some arthropods, it may be difficult to consistently and assist in this process by identifying patterns that stand out correctly identify the morphological criteria used to group speci- from noise, but do not, in themselves, discriminate biological mens into morphs or degrees. For example, precise counting of from taphonomically induced patterns. That task requires con- thoracic segment numbers can be difficult, especially during the sideration of whether patterns observed mimic the expectations meraspid period. Comparison with isolated (meraspid) pygidia of taphonomically induced variance. of approximately the same size range may be necessary for rec- ognizing and describing the articulation separating these regions Description and interpretation.—In approaching the study of at different stages. Any criteria used to identify articulations, and the ontogeny of fossil species, it is important, as far as thus the partition of the trunk into thoracic and pygidial seg- possible, to separate the description of fossils from their ments, should be described. developmental interpretation. A final note about the term “segment.” The description of Intraspecific data may show variation in size and shape, trilobite segmentation (as either pattern or process) is typically including in discrete characters, such as segment numbers. Infer- limited to a dorsal view of the exoskeleton: what is actually ring the ontogenetic process that produced the observed pattern observed is the subdivision of the dorsal exoskeleton into scler- of variation is an exercise in probabilistic inference because dif- ites, or tergites, including both the articulated tergites of the ferent processes can commonly produce the same pattern. With thorax and their non-articulated serial homologues within the respect to trilobite development, terms such as morph, degree, cephalon (where discernible) and in the pygidium. Pragmatic- segment, and tagma are morphological terms used to describe ally, in most papers as well as herein, the term segment is applied the phenotypic condition of a specimen. In contrast, terms to all these (articulated or not) serially homologous exoskeletal such as meraspid, holaspid, anamorphic, epimorphic, stage, units (Hughes et al., 2006). However, because in some arthro- and instar are developmental terms and, as such, can be pods there is a mismatch between dorsal and ventral segmental employed only after a given ontogenetic interpretation of the patterns (Fusco and Minelli, 2013), and the developmental pro- data has been made explicit and justified. In early descriptions cesses forming ventral and dorsal serially homologous struc- of trilobite ontogeny, such categorical differences had limited tures can operate independently (Janssen et al., 2004), the importance because the principal aim was to reveal the broad term segment should not be interpreted as referring to either a outline of how trilobites developed. However, as more subtle modular morphological unit of the whole body or to a develop- aspects of ancient developmental control are dissected, categor- mental unit of the main body axis (Fusco, 2008). ical differences become more important. For arthropod fossil species, which grew in a stepwise fash- Stage assignment.—In arthropods, postembryonic growth ion, ontogenetic reconstruction often starts from seeking pat- occurs mainly in a stepwise manner, in pace with the terns in the variation of form that allow us to partition the occurrence of ecdysis (Minelli and Fusco, 2013), and it seems study sample in a number of distinct morphological categories: natural to describe the ontogeny based on successive stages. any such categories of form are referred to as morphs. Morphs However, it should be noted that the assignment of specimens are established on a strictly descriptive basis, which considers to developmental stages is a kind of inference that is not the state of a discrete character, or of a combination of several always feasible, because stages may lack unique size range or discrete characters (e.g., the number of thoracic and pygidial morphological markers that distinguish them. segments, the latter counted, conventionally, to exclude the ter- Formally, two main types of ontogenetic morphometric minal piece). Conversely, the inference that one or a subset of data may be obtained from fossil series of molting animals, those morphs represents one sequential ‘step’ in development, cross-sectional and mixed cross-sectional data (Cock, 1966). known in arthropods as a stage or instar, is an interpretative Cross-sectional data are those for which assignment of a given undertaking that must not be confused with the prior, descriptive specimen to a certain developmental stage can be done with
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confidence on the basis of some morphological criterion (e.g., developed. Where more than a single developmental pathway the number of thoracic segments in immature trilobites, when apparently existed within a taxon, alternative segmentation sche- appropriate, or membership in a distinct size class). Mixed dules can be presented (e.g., Hughes et al., 2006, fig. 5A). Such cross-sectional data are those for which a criterion of stage diagrams may also illustrate competing developmental hypoth- assignment is not available, which is often the case in trilobite eses (Hou et al., 2015, fig. 7). specimens with the mature number of segments, or when distinct Data analysis might produce an ontogenetic hypothesis for a sizes classes are not evident. Both types of data can be used in taxon under study that a researcher could decide to represent studies of fossil ontogeny (e.g., Fusco et al., 2016; Hopkins, through some kind of graphical schematization. There can be sev- 2020), but each requires different processing. When a criterion eral reasons for this: to be consistent with the fact that not all the for stage assignment is available, both relative (allometric) and different details of the hypothesis may have the same evidentiary absolute (stage-based) growth analyses are possible. When support, to suitably highlight a specific aspect of development, or this is not the case, only size-related shape changes can be simply to provide a sketch of the inferred ontogeny. Figure 1 con- investigated (e.g., Holmes et al., 2020b), with no possibility to trasts two different schematizations (Fig. 1.2, 1.3)ofthesame separate static (i.e., within-stage) and ontogenetic (i.e., between- ontogenetic hypothesis (Fig. 1.1)forthesegmentationofan stages) allometry (Klingenberg, 2016). imaginary trilobite species, based on observed or conjectured The interpretation of stages is a critical step in any ontogen- morphs. Observations of this species suggest that there were mul- etic analysis of absolute growth, and care should be paid to justi- tiple alternative developmental pathways, of which the pathways fying any particular staging hypotheses. Justification must be drawn in Figure 1.1 are plausible candidates. The scheme based on some kind of evidence. For example, if using segment depicted in Figure 1.2 is made by simply arranging the observed numbers (either in the thorax, or in the trunk), the resulting per- morphs firstly by degree and then by the number of pygidial seg- stage growth rates or intra-stage size variation should exhibit ments. This generates a confusion between morphs (segmental some properties such as (proportional) regularity among stages. condition) and stages, and conflates the development of individ- Alternatively, a given staging hypothesis can be supported on ual trilobites with the pattern of variation in the sample as a the basis of specific morphological features (e.g., exoskeletal whole. If read as the ontogeny of an individual trilobite, as is ornament) that are seemingly added sequentially from stage to likely, the scheme clearly does not correspond to the ontogenetic stage. Stage assignments also can be made using a criterion of hypothesis one aims to depict (Fig. 1.1). On the contrary, the size and/or shape clustering (e.g., with respect to the cranidium), scheme depicted in Figure 1.3, while showing only one (presum- although reliable assignment of individuals to particular stages is ably the most common) of the six segmentation pathways that often only possible for the earliest stages. Methods for identifying individual trilobites could have followed in this case, is consistent instars based on size data were reviewed by Webster (2015). with the hypothesized ontogeny in Figure 1.1. Information about Whether based on qualitative, quantitative-discrete, or the existence of variation in the pattern of segment addition (and quantitative-continuous characters, it is always possible (and thus in the number of pygidial segments at each stage), and the often the case) that reliable stage assignment is only feasible for existence of more than one pattern of segment addition, can be a subset of specimens and/or stages. Beyond that subset, investi- conveyed with the legend or with other illustrations. gation must switch from cross-sectional data analysis to analysis In the theoretical example given above (also in Dai et al., of mixed cross-sectional data. As a result, certain analyses com- 2017, fig. 7), directly equating the morphological pattern monly apply only to particular portions of the ontogeny. observed with the developmental sequence shown in Figure 1.2 Choice of the staging criteria should also consider the entails an unlikely process of intermittent loss of trunk segments study’s objectives. For example, if the focus of a particular between instars. Such a pattern is not observed in extant arthro- study is to resolve the ontogenetic dynamics of segment release, pods, and is most unlikely to have occurred in trilobites. The then stage assignment should be made using a criterion inde- same potential problem exists with some other published seg- pendent of the number of segments (e.g., a size-clustering mentation schedules for polymerid trilobites in which two based on cranidial size), otherwise the possibility of identifying instars per meraspid degree are shown (e.g., Dai et al., 2014, within-stage variation in thoracic segment number is precluded. fig. 6; Lei, 2016, fig. 10; Du et al., 2020, fig. 9). Although mul- Conversely, if the focus is to determine per-stage growth rate and tiple instars evidently did occur within the earliest part of the its variation, a criterion of stage assignment independent of any meraspid period in many trilobites, before the release of any assumptions about growth patterns should be adopted. Whatever freely articulating segments in the thorax (e.g., Zhang and Clark- the case, the effects of potential confounding factors in stage son, 1993), it should be reiterated that published accounts of assignment on the results should be discussed. multiple meraspid instars in those trilobites with functional thor- acic segments are putative developmental scenarios, and remain Illustration of the inferred ontogeny.—Because illustrations hypotheses to be tested using independent evidence (also see effectively present and transmit interpretations, particular care Hou et al., 2015, p. 508–511). should be taken in the preparation of any diagrammatic representation of the inferred ontogeny and the legend that Use of nomenclature.—In describing the ontogeny of an extinct accompanies it. species, the developmental nomenclature of the extant group to Following McNamara et al. (2003) and Minelli et al. which the fossil taxon belongs, or is closely related, should be (2003), segmentation schedule diagrams have become common applied as far as possible. in studies of trilobite ontogeny. Their purpose is to illustrate the Careful application of the developmental nomenclature of ontogenetic pathway by which a representative individual extant organisms provides the best comparative framework
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available from which to highlight both the similarities and dif- the US National Science Foundation grant EAR-1849963 and ferences between the ontogenies of living and extinct forms by the Fulbright Academic and Professional Excellence (see Hughes et al., 2006, p. 621, on the misinterpretation of tri- Award 2019 APE-R/107 kindly hosted at the Indian Statistical lobite development in a major text book in invertebrate zoology Institute, Kolkata. XZ is funded by Natural Science Foundation resulting from trilobite-specific terminology use). To this aim, of China (Grant Nos 41621003, 41890840 and 41930319) and reference to a standard manual of the group can be of help the 111 Project (D17013). The paper is a contribution to (e.g., for arthropods, see Minelli et al., 2013). IGCP 668 project “The stratigraphic and magmatic history of For instance, in arthropods the term “segmentation” is used Early Paleozoic equatorial Gondwana and its associated evolu- to describe both a morphological feature (translational body tionary dynamics.” symmetry) and the developmental process that generates it (Fusco and Minelli, 2013; Dai et al., 2017; Du et al., 2020). References The term “somitogenesis,” which is sometimes used in trilobite literature to indicate the appearance of new segments in the trunk Barrande, J., 1846, Notice Préliminaire sur le Système Silurien et les Trilobites fi prior to the onset of maturity (McNamara et al., 2006; Lei, de Bohême: Leipzig, Hirsch eld, 96 p. Cock, A.G., 1966, Genetical aspects of metrical growth and form in animals: 2016), is not normally used for the segmentation process in Quarterly Review of Biology, v. 41, p. 131–190. extant arthropods, especially in the case of post-embryonic seg- Dai, T., Zhang, X.-L., and Peng, S.-C., 2014, Morphology and ontogeny of Hunanocephalus ovalis (trilobite) from the Cambrian of South China: Gon- mentation (anamorphic development). What is observed during dwana Research, v. 25, p. 991–998. post-embryonic development in anamorphic arthropods is the Dai, T., Zhang, X.-L., Peng, S.-C., and Yao, X.-Y., 2017, Intraspecific variation appearance of new exoskeletal segmental units in the posterior of trunk segmentation in the oryctocephalid trilobite Duyunaspis duyunen- sis from the Cambrian (Stage 4, Series 2) of South China: Lethaia, v. 50, of the trunk, which may not coincide with other aspects of seg- p. 527–539. ment generation. This is the reason why in fossils phrases such Du, G.-Y., Peng, J., Wang, D.-Z., Wen, R.-Q., and Liu, S., 2020, Morphology as “segment appearance” or “segment morphological expres- and trunk development of the trilobite Arthricocephalus chauveaui from the ” “ Cambrian Series 2 of Guizhou, South China: Historical Biology, v. 32, sion should be preferred to phrases such as segment gener- p. 174–186. ation” or “segment proliferation.” Feldman, R.M., 1989, Whitening fossils for photographic purposes, in Feld- Similarly, “tagmosis” is used to indicate a morphological man, R.M., Chapman, R.E., and Hannibal, J.T., eds., Paleotechniques: The Paleontological Society Special Publication, Knoxville, University of characteristic (a form of body organization) as well as the devel- Tennessee, v. 4, p. 342–346. opmental processes that generate it (Fusco and Minelli, 2013). Fusco, G., 2008, Morphological nomenclature, between patterns and processes: Although there is little consensus on how tagmata should be segments and segmentation as a paradigmatic case, in Minelli A., Bonato L., fi and Fusco G., eds., Updating the Linnaean Heritage: Names as Tools for de ned in modern arthropods, the process of tagmosis (also called Thinking About Animals and Plants: Zootaxa, v. 1950, p. 96–102. tagmatization) in some way describes the ontogenetic subdivision Fusco, G., and Minelli, A., 2013, Arthropod body segments and tagmata, in of the main body axis into major morpho-functional units. In Minelli, A., Boxshall, G., and Fusco, G., eds., Arthropod Biology and Evo- lution: Molecules, Development, Morphology: Berlin, Heidleberg, Springer trilobites and their close relatives, the peculiar process of their Verlag, p. 197–221. development known as release, which involved the formation of Fusco, G., Hughes, N.C., Webster, M., and Minelli, A., 2004, Exploring devel- a new functional segment articulation at the posterior end of the opmental modes in a fossil arthropod: growth and trunk segmentation of the trilobite Aulacopleura konincki: American Naturalist, v. 163, p. 167–183. anterior-most segment of the pygidium, has been described as Fusco, G., Hong, P.S., and Hughes, N.C., 2016, Axial growth gradients across tagmosis (e.g., McNamara et al., 2006). Segment release is not the postprotaspid ontogeny of the Silurian trilobite Aulacopleura koninckii: observed in extant arthropods, but in many anamorphic taxa Paleobiology, v. 42, p. 426–438. “ ” “ ” Hartnoll, R.G., and Bryant, A.D., 1990, Size-frequency distributions in decapod segment appearance followed by maturation at a later stage Crustacea—the quick, the dead and the cast-offs: Journal of Crustacean (e.g., with regard to the formation of the appendages) occurs Biology, v. 10, p. 14–19. and this pattern is not generally regarded as a change in tagmosis. Holmes, J.D., Paterson, J.R., and García-Bellido, D., 2020a, The post- embryonic ontogeny of the early Cambrian trilobite Estaingia bilobata As the segmental boundary between thorax and pygidium shifted from South Australia: trunk development and phylogenetic implications: posteriorly during part of ontogeny (defining the meraspid Papers in Palaeontology. doi:10.1002/spp2.1323. period), the trilobite thorax and pygidium have been suggested Holmes, J.D., Paterson, J.R., and García-Bellido, D., 2020b, The trilobite Red- lichia from the lower Cambrian Emu Bay Shale Konservat-Lagerstätte of to be parts of one tagma, the trunk (Minelli et al., 2003). South Australia: systematics, ontogeny and soft-part anatomy: Journal of Systematic Palaeontology, v. 18, p. 295–334. Hong, P.S., Hughes, N.C., and Sheets, H.D.S., 2014, Size, shape and systema- Conclusions tics of the Silurian trilobite Aulacopleura koninckii: Journal of Paleontology, v. 88, p. 1120–1138. Here we do not present a comprehensive review of the method- Hopkins, M.J., 2020, Ontogeny of the trilobite Elrathia kingii (Meek) and com- parison of growth rates between Elrathia kingii and Aulacopleura koninckii ology of ontogenetic analysis of articulated trilobites and their (Barrande): Papers in Palaeontology. doi:10.1002/spp2.1331. relatives, but rather address specific topics that may help the grow- Hou, J.-B., Hughes, N.C., Lan, T., Yang, J., and Zhang, X.-G., 2015, Early post- ing number of case studies to be of best comparative value. It is embryonic to mature ontogeny of the oryctocephalid trilobite Duodingia duodingensis from the lower Cambrian (Series 2) of southern China: Papers our hope that application of this methodology can further advance in Palaeontology, v. 1, p. 497–513. understanding of the developmental basis of ancient evolution. Hughes, N.C., 2003, Trilobite tagmosis and body patterning from morpho- logical and developmental perspectives: Integrative and Comparative Biol- ogy, v. 41, p. 185–206. Acknowledgments Hughes, N.C., Minelli, A., and Fusco, G., 2006, The ontogeny of trilobite seg- mentation: a comparative approach: Paleobiology, v. 32, p. 602–627. Janssen, R., Prpic, N., and Damen, W.G.M., 2004, Gene expression suggests We thank all editors, L. Amati, and an anonymous reviewer for decoupled dorsal and ventral segmentation in the millipede Glomeris mar- their helpful comments. NCH’s contribution was supported by ginata: Developmental Biology, v. 268, p. 89–104.
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